Moar Ediacara – the rangeomorphs of Mistaken Point

by Matthew Cobb

I just realised that the perfect counter-point to yesterday’s post about the Ediacara is this excellent Palaeocast podcast with Dr. Alex Liu of the University of Cambridge, who has been studying one of the best-known Ediacaran fossil formations, at Mistaken Point in Newfoundland. This link to the podcast will also take you to a set of fantastic photos by Dr Liu.

You should also read this great post on Mistaken Point by Tony Martin from his terrific blog Life Traces of the Georgia Coast.

The Ediacaran rocks are named after Ediacara in Australia, but there are a number of other key sites around the world that yield bizarre and enigmatic fossils. Mistaken Point – a deep tropical sea bed 565 million years old – is particularly important as it contains many fossils of rangeomorphs. Rangeomorphs look vaguely like ferns, but a) they show a fractal-like structure that is utterly different from a fern and b) they were (apparently) found in the deep ocean. This latter point indicates that they could not have photosynthesised and were therefore probably some kind of animal. With their high surface-area to volume ratio, they could have been osmotrophs, directly absorbing carbon, or perhaps they were some kind of early cnidarian, like a sea pen.

One thing the Mistaken Point biota were not, however, was terrestrial. And here’s a further indication: a trace of an animal, about 1cm wide, moving along the sea-floor from left to right (Tony Martin’s pal Paleontologist Barbie is there for scale):

Caption and photo from here: “A surface trail, probably made by a < 1 cm wide animal moving along the seafloor about 565 mya. The animal moved from left to right, which is indicated by the crescentic ridges inside the trail, which open in the direction of movement. The crecentic ridges in the interior of the trail may represent marks where the basal disc of a anemone-like animal pushed against the surface as it moved. Even more interesting, the arrow points to an oval impression, which may be a resting trace that shows the approximate basal diameter of the tracemaker. What was the tracemaker? It’s currently identified as a small anemone, which is based on modern traces. Neoichnology rules! (Photograph by Anthony Martin, taken at Mistaken Point, Newfoundland)”

Martin Brasier, author of the marvellous Darwin’s Lost World (highly recommended) has recently published an article (Open Access! Hooray! – pics below taken from here) in which he has tried to show how rangeomorphs grew, by looking at hundreds of fossils. Here are some examples of what he’s been looking at, followed by his model for how a couple of these life-forms may have grown. By focusing just on the ‘architecture’ (his term) of the rangeomorphs, he and his colleagues think they have provided a new framework for classifying these organisms.

Finally, here’s Brasier from a few years back advertising his excellent book and briefly explaining how he got hooked on pre-Cambrian rocks and their importance for our understanding of evolution:

I was working in Canada last year and was trying to persuade the people paying us to let me (and my colleague) hire a car for a “jolly” down to Mistaken Point, instead of spending 9 hours a day sitting in a helicopter departure lounge, looking at the inside of a fog bank. Would have been fun – well, “more fun” – but the customers insisted on us staying within 20 minutes of the heliport, just in case there was a weather window. [SIGH]
Next year maybe?
(The rig got hit by a 22m wave on that job. Not fun, particularly since we were a couple of miles from the Ocean Ranger site. And we had the 60-odd sq.km of the Petermann II Iceberg bouncing down the Labrador coast in our direction too. Deee-light-ful.
To my abiding shame, I was “drug up proper like!” less than an hour from Charnwood … but I’ve never been there. Not that I’d be allowed to take my hammer with me anyway (as if I really needed one [GRIN]).

Thank you, especially Brazier et al’s paper on rangeomorph growth as I was prompted into a question relating to that.

– These rangeomorphs, like many other Ediacarans like the Dickinsonia showed yesterday and the Spriggina gravelinspector referred to then, has one or several organized polar growth axes that coordinates growth as a glide reflection of subunits.

– A polarized growth axis also seems to appear in Parvancorina whether or not it was a hold-fast.

Why do you evolve a growth axis?

If I look at today’s plants, when they do that they are typically competing for light.

If I look at something similar to the rangeomorphs, which all had a basal disk or (it seems to me) a more cryptic initial growth center, they want to reach up in convective streams for nutrients or food.

And if I look at animals, they want to go or reach places in a more coordinated manner (so not today’s placozoa or sponges). Or perhaps I should say “polarized” manner.

I’m thinking of Dickinsonia, Spriggina et cetera. Wouldn’t the lateral growth axis relative to the absence of hold-fast predict they were moving organisms?

[I would also like to than gravelinspector on comments yesterday leading into the question of stasis, related to the increase of environmental oxygen or not. It is something that needs to be handled in astrobiology when assessing habitable exoplanets’ atmospheres, so I was going to delve into that and appreciate the input.

I also remember Hazen’s lecture here a few years ago, where he described a period of then apparent stasis in the coupled mineral evolution. It may or may not be correlated to the FIB redox cycling or its aftermath, I can’t remember. But there are at least one of these apparent or real phenomena to understand.]